1. Field of the Invention
The present invention relates to circuits for restoring logic levels of bits transmitted by a signal asynchronously and in series. The present invention more particularly relates to cases in which the transmission signal periodically has synchronization bursts for periodically readjusting the processing circuit.
2. Discussion of the Related Art
FIG. 1 represents an exemplary waveform of a signal S for transmitting teletext information. The teletext information is transmitted during non-visible lines of a television picture. Thus, each "line" of the teletext signal is preceded by a line synchronization pulse Hs. Shortly after this pulse Hs, a synchronization burst SB starts which has a mean level higher than the level preceding the burst. This burst includes, for example, eight sine wave periods. The half-period of these sine waves corresponds to the nominal transmission duration of the bits that follow. Thus, the effective signal Tx for transmitting the bits starts immediately after the synchronization burst SB. In this example, the transmission starts with the bits 11100100100110110.
Depending upon the reception conditions, a teletext signal may be noisy and have a variable width and phase. With the synchronization burst SB at the beginning of each teletext line, the processing circuit can periodically readjust the phase of its reception frequency and a discrimination threshold for differentiating the high logic levels from the low logic levels during the effective transmission of signal Tx. U.S. Pat. No. 5,136,382 describes such a processing circuit. In this circuit, the discrimination threshold follows the mean change over time of signal S until a portion of the synchronization burst has passed, so that the discrimination threshold establishes at the mean value of the synchronization burst. Then, the discrimination threshold is held constant during the effective transmission of signal Tx.
In order to determine when the discrimination threshold is to be fixed, it is necessary to detect the occurrence of the synchronization burst SB. Indeed, the time interval separating this burst from the synchronization pulse Hs randomly varies about a nominal value with a relatively large tolerance. For this purpose, for example, the crossing of the discrimination threshold by signal S is detected after the synchronization pulse Hs. Since the discrimination threshold follows the mean change over time of signal S, this discrimination threshold is lower than the sine waves of the burst at the beginning of the synchronization burst. To decrease the possibility of spurious detections caused by the noise of signal S, this detection is enabled as late as possible after the synchronization pulse Hs and only if signal S satisfies a criterion, for example, if the discrimination threshold is exceeded for a predetermined time.
Despite the precautions taken, it may happen, if noise is excessive, that this detection criterion of the synchronization burst is satisfied before the effective occurrence of the burst. As a consequence, the discrimination threshold is fixed at a wrong value, which causes erroneous interpretations of the bits of portion Tx of the signal.
Moreover, depending upon the reception conditions, the teletext signal may be subject to high frequency losses, especially if the teletext signal is Nyquist filtered for reducing spurious echo phenomena (inter-symbol modulation).
FIG. 2 represents an exemplary waveform of the portions SB and Tx of the signal of FIG. 1 subject to high frequency losses. The synchronization burst SB, of relatively high frequency, is substantially attenuated. However, because of the symmetry of the burst SB, its mean value is not altered, which allows a suitable establishment of the discrimination threshold Vref Problems may arise especially when transmitting isolated 1's preceded and followed by at least two 0's, as for the sixth and ninth transmitted bits shown in FIG. 2, or when transmitting isolated 0's preceded and followed by at least two 1's, as for the fourteenth and seventeenth transmitted bits shown in FIG. 2. Indeed, by transmitting several consecutive bits at the same state, the signal may vary for the duration of these several bits, which enables the signal to reach its maximum excursion. In contrast, if the bit transmitted immediately after is at a different isolated state, the signal can only vary for the duration of a single bit, which generally prevents it from reaching its maximum excursion. In this case, as shown for the sixth, ninth, fourteenth and seventeenth bits of FIG. 2, signal S hardly exceeds the discrimination threshold Vref It is then highly probable, especially because of noise, that erroneous states of the bits are sampled. In addition, in some cases, signal S does not reach the threshold Vref